In reply to BnB:
Both Siabod and Hebog aren't walks they're flogs.
Hours to get to the top and then what?
Back down again because they aren't joined to anything else.
All made worse by being able to see the pub tantalisingly out of reach.
As for other peoples suggestions, Rhinogs, Arrans, Pen Lythrig Y Wrach, etc - wastelands, cold, wet, god forsaken wastelands.
Don't encourage anybody to visit these places, go to Mount Snowdon instead !